Placement Studio

Speed Maths & Mental Aptitude Lab

Kill the calculation bottleneck in TCS NQT / Infosys / Cognizant tests. Learn each shortcut, practise it, then survive the 60-second reaction drill.

60-second reaction drill

Rapid mental-math cards across all 8 shortcuts. Answer as many as you can in 60 seconds.

Multiply numbers near 100 / 1000 in one line

Rule: (100+a)(100+b) = 100×(100+a+b) + a×b — add the deviations, append the product of deviations.

Example: 96 × 97: deviations −4, −3 → (96−3)=93 | (−4)(−3)=12 → 9312

85², 115²… instant

Rule: (10t + 5)² = t(t+1) × 100 + 25 — multiply t by t+1, append 25.

Example: 85²: t = 8 → 8×9 = 72 → 7225

1/7 = 14.28% and friends, instantly

Rule: Memorise 1/2 → 1/20 as percentages; every percentage question becomes a fraction multiply.

Example: 16.67% of 420 = (1/6) × 420 = 70

Check 3, 7, 11, 13 without dividing

Rule: 3: digit sum. 7/13: double-unit or 100×rest+last tricks. 11: alternating digit sum.

Example: 2,03,617 ÷ 7? Alternating blocks of 3: 617 − 203 = 414 = 7×59 → divisible.

Cyclicity of 2,3,7,8 in 4 steps

Rule: Unit digits cycle: 2→2,4,8,6; 3→3,9,7,1; 7→7,9,3,1; 8→8,4,2,6. Use (exp−1) mod cycle length.

Example: unit digit of 3^47: cycle 3,9,7,1; (47−1) mod 4 = 2 → 7

Century codes + odd days

Rule: Sakamoto: t = [0,3,2,5,0,3,5,1,4,6,2,4]; for Jan/Feb use year−1; dow = (y + y/4 − y/100 + y/400 + t[m−1] + d) mod 7.

Example: 15-Aug-1947 → (1947 + 486 − 19 + 4 + 3 + 15) mod 7 = 5 → Friday

Angle between hands in one step

Rule: Angle = |30h − 5.5m|; hands coincide every 720/11 minutes.

Example: 3:40 → |90 − 220| = 130°

Mix two prices to hit a target mean

Rule: Cheaper : dearer = (dearer − mean) : (mean − cheaper).

Example: Mix ₹60 and ₹80 rice to sell at ₹72 → 8 : 12 = 2 : 3